1. Field of the Invention
The present invention relates to a current output circuit in which an amount of current is adjusted through zapping.
2. Description of the Related Art
In many cases, the manufacture of various circuits requires a final adjustment process. In particular, in semiconductor integrated circuits or the like, variations among elements cannot be completely eliminated, and thus, a product requires adjustment of characteristics after the completion of manufacture.
Various methods are employed for this final adjustment, including adjustment of an amount of current within an internal circuit through zapping. In this zapping process, for example, a zapping terminal to which a zapping diode is connected is provided and a predetermined voltage is applied to the zapping terminal to induce breakdown of the zapping diode. Provision of a transistor which is switched on and off by the zapping diode enables adjustment of an amount of current of a constant current source or the like within an internal circuit.
A zapping circuit described above is embodied by various circuits, including a structure disclosed in Japanese Patent Laid-Open Publication No. 2002-261243.
In many transistors which are switched on and off through zapping, an amount of current flowing through the transistor when the transistor is switched on is an adjustment current that is added to or reduced from a reference current. For example, in the above-identified Japanese Patent Laid-Open Publication No. 2002-261243, ON/OFF states of a plurality of adjustment current transistors are controlled in order to control the overall amount of current. Therefore, the amount of current flowing through the adjustment current transistor when the transistor is ON is an important parameter.
In conventional devices, the transistor for inducing flow of the adjustment current is generally connected in series to a resistance, and the magnitude of the adjustment current flowing through the transistor is set by the magnitude of the resistance. However, because the adjustment current transistor is generally switched fully on, Vce becomes small and the transistor becomes saturated. Therefore, the magnitude of the adjustment current is affected not only by the resistance of the resistor, but also by the ON-resistance (emitter resistance) of the adjustment current transistor. The ON-resistances of saturated transistors are significantly affected by variations among transistors, raising a problem that the adjustment currents vary. In addition, the ON-resistance of a transistor has temperature characteristics, and compensation of these characteristics has been difficult.